1. Field of the Invention
The present invention relates to a stator of a drive motor which is equipped for an electrically operated valve used for controlling a flow rate of a cooling medium which circulates in a refrigeration cycle.
2. Description of the Conventional Art
JP-A-2009-287663 discloses this kind of electrically operated valve.
The electrically operated valve is provided with a valve mechanism within a valve main body, and a sealed container called as a can is attached to an upper portion of the valve main body. A rotor of a driving motor is equipped in an inner portion of the can, and a turning force of the rotor is converted into a motion in a linear direction by a screw mechanism so as to operate a valve member. The valve member linearly moves so as to control a valve opening degree. A stator of the driving motor is detachably mounted to an outer peripheral portion of the can.
The stator is structured such that a stator assembly having a coil wound around magnetic pole teeth of a yoke is set within a metal mold for a resin molding, and a molten resin is supplied into the metal mold, whereby the stator assembly is integrally formed by a synthetic resin.
In the electrically operated valve mentioned above, in order to make a magnetic gap between the stator and the rotor as small as possible, the magnetic pole teeth of the yoke arranged within the stator are exposed to an inner portion of the stator, however, the other portions than a portion which is opposed to the can in the magnetic pole teeth are molded by the resin, so as to prevent a moisture from intruding into the coil within the stator from an external portion.
In JP-A-2009-287663, there is disclosed a technique which prevents water from intruding into a portion between the can and the stator, by providing a seal member between a lower portion of the stator and a lower portion of the valve main body or the can, in order to further securely prevent the moisture intrusion.
In JP-A-2000-262024, there is disclosed an example of a molding method of the stator for the electrically operated valve. As mentioned above, since it is necessary for the stator to be molded by the resin in the other portions than the portion which is opposed to the can in the magnetic pole teeth, a yoke accommodating a coil wound around a bobbin is structured such as to be injected the resin from a plurality of gates.
FIGS. 5 to 7 show a structure of a stator of a drive motor for a conventional electrically operated valve. A coil 140 wound around a bobbin 130 is retained from above and below by an outer yoke 120 which is provided with magnetic pole teeth 120a, and an inner yoke 122 which is provided with magnetic pole teeth 122a. Further, a stator assembly 110 is constructed by bringing two sets of retaining bodies into contact with each other so as to be opposed to each other and positioning them. In the stator assembly 110, a resin mold 200 is formed around the stator assembly 110 so that only inner surfaces (inward directed surfaces of the stator) of the magnetic pole teeth 120a and 122a, and a lead pin 141 drawn out of the coil 140 are exposed.
The resin mold 200 is formed, for example, according to the following manner. In other words, the stator assembly of a stator 100′ is set within a metal mold which is not illustrated, and the resin is injected into the metal mold from an upper gate G1 and lower gates, for example, three lower gates G2, G3 and G4.
The stator assembly 110 is integrally formed with the resin mold 200 and the stator 100′ is finished.
As mentioned above, in an inner peripheral surface of the stator 100′, it is necessary to circulate the resin in the other portions than the inner surfaces of the magnetic pole teeth 120a and 122a. Accordingly, it is necessary for the resin injected from the gates provided in an outer periphery of the stator to be guided to the inner peripheral surface side of the stator after passing through a complicated internal structure portion of the stator assembly.
Therefore, in the conventional manufacturing method, since the molding operation is carried out in the multiple points of gates (for example, four gates) which are provided in the upper and lower sides of the metal mold, so that the molten resin is circulated around the stator, a weld line WL is generated in an outer peripheral portion of the resin mold 200.
The weld line is a re-melting position of the resin in the resin molding, and a mechanical strength of the re-melting position is lowered in comparison with the other molded surfaces.
In the case that an electrically operated valve using the stator is mounted, for example, to a motor vehicle or a marine vessel, an environmental temperature range in an air temperature and a low temperature becomes large, and a temperature fluctuation do damage to the coil resin part. Therefore, a crack is generated in the weld line portion, and there is a risk that a deterioration of an insulating performance is caused by a water intrusion from the crack position.